Use blink-setting for colorscheme on Qt 5.15+

On Qt 5.15+, `--force-dark-mode` does not set the preferred colorscheme.
A blink-setting is used instead to set the preferred colorscheme.

The `--force-dark-mode` flag is only set for Qt 5.14. All later versions
will use the blink-setting flag.

(cherry picked from commit 314c81b24eb9f9457ccdb5c682d718cf24f24a76)

# Conflicts:
#	qutebrowser/browser/webengine/darkmode.py
#	tests/unit/browser/webengine/test_darkmode.py

3 files changed, 36 insertions, 11 deletions

diff --git a/qutebrowser/config/qtargs.py b/qutebrowser/config/qtargs.py
index fbb9fe908..b42484a08 100644
--- a/qutebrowser/config/qtargs.py
+++ b/qutebrowser/config/qtargs.py
@@ -79,6 +79,10 @@ def _darkmode_prefix() -> str:
 
 def _darkmode_settings() -> typing.Iterator[typing.Tuple[str, str]]:
     """Get necessary blink settings to configure dark mode for QtWebEngine."""
+    if (qtutils.version_check('5.15', compiled=False) and
+            config.val.colors.webpage.prefers_color_scheme_dark):
+        yield "preferredColorScheme", "1"
+
     if not config.val.colors.webpage.darkmode.enabled:
         return
 
@@ -330,12 +334,18 @@ def _qtwebengine_settings_args() -> typing.Iterator[str]:
             False: '--autoplay-policy=user-gesture-required',
         }
 
-    referrer_setting = settings['content.headers.referer']
-    if qtutils.version_check('5.14', compiled=False):
+    if (qtutils.version_check('5.14', compiled=False) and
+            not qtutils.version_check('5.15', compiled=False)):
+        # In Qt 5.14, `--force-dark-mode` is used to set the preferred
+        # colorscheme. In Qt 5.15, this is handled by a blink-setting
+        # instead.
         settings['colors.webpage.prefers_color_scheme_dark'] = {
             True: '--force-dark-mode',
             False: None,
         }
+
+    referrer_setting = settings['content.headers.referer']
+    if qtutils.version_check('5.14', compiled=False):
         # Starting with Qt 5.14, this is handled via --enable-features
         referrer_setting['same-domain'] = None
     else:
diff --git a/tests/helpers/utils.py b/tests/helpers/utils.py
index 46787391d..64779dfc6 100644
--- a/tests/helpers/utils.py
+++ b/tests/helpers/utils.py
@@ -52,6 +52,9 @@ qt514 = pytest.mark.skipif(
 skip_qt511 = pytest.mark.skipif(
     qtutils.version_check('5.11'), reason="Needs Qt 5.10 or earlier")
 
+qt515 = pytest.mark.skipif(
+    not qtutils.version_check('5.15'), reason="Needs Qt 5.15 or newer")
+
 
 class PartialCompareOutcome:
 
diff --git a/tests/unit/config/test_qtargs.py b/tests/unit/config/test_qtargs.py
index 69aef8b97..644765a58 100644
--- a/tests/unit/config/test_qtargs.py
+++ b/tests/unit/config/test_qtargs.py
@@ -317,20 +317,20 @@ class TestQtArgs:
         else:
             assert arg in args
 
-    @pytest.mark.parametrize('dark, new_qt, added', [
-        (True, True, True),
-        (True, False, False),
-        (False, True, False),
-        (False, False, False),
+    @pytest.mark.parametrize('dark, qt_version, added', [
+        (True, "5.13", False),
+        (True, "5.14", True),
+        (True, "5.15", False),
+        (False, "5.13", False),
+        (False, "5.14", False),
+        (False, "5.15", False),
     ])
     @utils.qt514
     def test_prefers_color_scheme_dark(self, config_stub, monkeypatch, parser,
-                                       dark, new_qt, added):
+                                       dark, qt_version, added):
         monkeypatch.setattr(qtargs.objects, 'backend',
                             usertypes.Backend.QtWebEngine)
-        monkeypatch.setattr(qtargs.qtutils, 'version_check',
-                            lambda version, exact=False, compiled=True:
-                            new_qt)
+        monkeypatch.setattr(qtargs.qtutils, 'qVersion', lambda: qt_version)
 
         config_stub.val.colors.webpage.prefers_color_scheme_dark = dark
 
@@ -591,6 +591,18 @@ class TestDarkMode:
             assert opt.restart, name
             assert opt.raw_backends == backends, name
 
+    @pytest.mark.parametrize('enabled, expected', [
+        # Disabled or nothing set
+        (False, []),
+
+        # Enabled in configuration
+        (True, [("preferredColorScheme", "1")]),
+    ])
+    @utils.qt515
+    def test_colorscheme(self, config_stub, monkeypatch, enabled, expected):
+        config_stub.set_obj('colors.webpage.prefers_color_scheme_dark', enabled)
+        assert list(qtargs._darkmode_settings()) == expected
+
 
 class TestEnvVars: